The year is 1925. Laura Bow is a young student attending the Tulane University in New Orleans, and an aspiring journalist. Her friend Lillian invites her to spend a weekend in a mansion belonging to her uncle, Colonel Henri Dijon, a veteran of Spanish-American war and the owner of a large, but decaying sugar plantation on a secluded island. During the dinner Laura meets members of Lillian's extended family and their alleged friends. As Laura begins to explore the mansion and talk to its inhabitants, she unravels old family feuds and realizes some of those people - if not all - have their own plans and schemes. It all gets much worse when strange accidents and murders start to occur. Laura must investigate those gruesome events and find the culprit.
The Colonel's Bequest is a detective mystery adventure game very similar in style and presentation to Agatha Christie's novels. The game world is confined to the island and consists of numerous interconnected areas, such as various rooms in the mansion, garden, smaller structures, etc. The player is free to explore much of the game's world right from the beginning, with only a few areas being initially inaccessible. The game's clock advances a quarter of an hour each time Laura performs an action pertaining to the plot. Characters have their own schedules and clues are often obtained by discretely observing their actions.
The player interacts with the environment by typing verb-object command combinations (such as "Look Garden", "Talk Colonel", etc.). Some of the most common actions have keyboard shortcuts. The gameplay largely focuses on exploration. Much of the plot stays in the background and can be uncovered by talking to the characters about various topics (mostly other people). There is an inventory and a few puzzles, but they play a much less prominent role in the game compared to other adventures by Sierra. It is possible to complete the game without having discovered the entire plot or even without having identified the murderer. There is no scoring system, but after the game is finished the player is given a detective ranking and told about story branches or other elements he might have missed.